In a VCR utilizing the NTSC format, for example, a color signal having the frequency of 3.58 MHz is converted to the signal at a lower frequency, and is then recorded on a video tape. A phase inversion is applied by a reproducing circuit, to the color signal during every other horizontal scanning period in order to eliminate cross-talk from adjacent tracks on the tape. Accordingly, during the reproducing operation, the color signal is re-converted to 3.58 MHz, and then added to the signal of the next horizontal scanning period by a comb filter in order to eliminate cross-talk.
FIG. 1 is a block diagram illustrating a known AGC circuit which is used for stabilizing the level of the reproduced color signal in a VCR and for eliminating cross-talk. In the AGC circuit of FIG. 1, a low frequency color signal CS which is read by a magnetic head from a video tape, is amplified by a variable gain amplifier 11; signal CS is inverted during every other horizontal scanning period before being supplied to amplifier 11. The amplified color signal CS1 is then converted by frequency converter 13, to a color signal CS2 having the baseband frequency of 3.58 MHz. Band-pass filter 15 extracts from signal CS2 only the baseband color signal CS3 and supplies it to comb filter 17. Comb filter 17 reduces, and preferably eliminates, the cross-talk components and supplies an output signal CS4 to both the next processing stage and a burst detector 19. In particular, comb filter 17 adds the inverted signal CS3 with the corresponding (non-inverted) CS3 signal of the next horizontal scanning period. This has the effect of eliminating cross-talk. Burst detector 19 detects the level of the burst signal included in signal CS4 and supplies amplifier 11 with a control signal for controlling its gain so the burst signal level remains constant. By this process, an AGC feedback loop is achieved for stabilizing the level of the reproduced color signal.
There is comparatively less cross-talk in the standard play (SP) mode of a VCR versus the cross-talk in its extended play (EP) mode. This is due to the width of the recording track, in the EP mode, being narrower than the track width in the SP mode. During the EP mode, signal levels of the reproduced color signals CS will vary greatly from horizontal period to horizontal period. For example, the color signal level during the previous horizontal scanning period may be twice that of the signal in the SP mode, while its level in the following horizontal scanning period may be approximately zero. In addition, as greater cross-talk components are included in signal CS, the level of output signal CS4 will decrease due to the removal of the cross-talk components by comb filter 17. Thus, in the above AGC feedback loop, signal CS must be amplified to a greater extent when its level is sufficiently high due to cross-talk.
Considering the cross-talk in the EP mode, the dynamic range of the circuits of FIG. 1 prior to comb filter 17 must have twice the level necessary for processing the color signal in the SP mode. During the SP mode, only half of the dynamic range of frequency converter 13 and band-pass filter 15 is used for processing the color signals. In other words, frequency converter 13 and band-pass filter 15 must have an unnecessarily large dynamic range for operation in the SP mode. As a result, the color signals processed by the circuit of FIG. 1 suffer disadvantages; for example, the signals are intolerant to disturbances such as carrier leaks in frequency converter 13, and noise in band-pass filter 15, particularly, if an active filter is utilized.
Moreover, since the AGC feedback loop includes comb filter 17 which has a large time delay, the response of the loop cannot be appreciably increased. As a result, the AGC circuit of FIG. 1 will likely produce a flicker along the upper portion of the screen.